Humanscale makes the M8 monitor arm that is rated for 40 lbs. I'm not sure which TV you have, but most new 55" TVs come under that without the stand. I am not sure if the arm has enough travel for re positioning the screen where you need though. I've seen it used for a 43" screen and it works well. I'll be picking one up for my sit/stand desk soon.
Ergotron also has an HX arm that is rated for 42 lbs. I've not seen it before, but it looks like it may work too.
Ergotron (and others) also make wall mounted arms that can accommodate 40+ lbs. I know you said desk mount, but if your desk is against a wall these can work too.
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